Shinnok is considered to be one of the most evil and one of the most powerful beings in the universe. Shinnok is a very powerful fallen elder god who is very vengeful.
Unlike Shao Kahn who relies on brute strength and force Shinnok relies more on sorcery and long forgotten, ancient artifacts. Shinnok is actually even more brutal and heartless than Shao Kahn.
He is the ruler of the netherrealm he possesses millions of years of knowledge and power, including the ability to perfectly impersonate any other beings of his choosing, as well as to transform into a giant immense demon. Shinnok demands from his followers unwavering faith and loyalty and will not hesitate to kill anyone who doubts or challenges him.
Shinnok fall from grace was because of his lust for power and mostly because of his desire to take earthrealm for himself.
At the dawn of time when all the realms were created, Shinnok was determined an desired to take over earthrealm and acquire immense power. However Shinnok could not enter earthrealm without the elder gods noticing his actions. So without defying and breaking the sacred rules completely Shinnok created a special amulet which weakened the dimensional barriers and entered earthrealm. During the war against his fellow gods Shinnok came into direct conflict with the thunder god Raiden.
During the final stages of this war, Raiden was forced to choose between sacrificing Earth's indigenous races, or ceding the rule of Earthrealm to Shinnok. The Elder gods intervened, and assisted Raiden in his struggle. Raiden defeated Shinnok, and stripped him of his amulet and his status as an Elder God. Shinnok was then banished to the deepest regions of the Netherealm. However, at the expense of Shinnok's defeat, the dimensional fibers of the worlds were severely weakened, and the saurians, Earth's indigenous life form at the time, were nearly exterminated.
While he was in the Netherrealm, Shinnok endured constant torture from the Netherrealm's overlord, Lucifer. Shinnok was at his mercy until he encountered a sorcerer, Quan Chi, who could travel through the realms at will. Quan Chi would aid Shinnok in overthrowing Lucifer and taking his throne, becoming ruler of the Netherealm. As payment for Quan Chi's efforts, Shinnok endowed him with incredible powers, making him arch-sorcerer of the Netherealm, and made him his most-trusted servant.
Shinnok began building his second army of darkness, and plotted to escape the Netherealm once more. In order to do so however, he would need his lost sacred amulet. After centuries of searching, Quan Chi eventually discovered the amulet's whereabouts for him. It had been in the possession of the Lin Kuei. Using the Lin Kuei warrior the elder sub-zero, Quan Chi recovered it, but unbeknownst to Shinnok, he created a flawed replica in its place which he delivered, keeping the original for himself. His attempt to invade Earthrealm came to naught as he was defeated by its champions and banished back to the Netherrealm by Raiden.
Shinnok set off the Battle of Armageddon by spreading word about the godlike power that could be gained from killing the elemental Blaze, then manipulated demigod brothers Taven and Daegon in their quest in order to guide them both along opposite paths so they would fight one another instead of working together. Through all this, he was even savvy enough to use a clone instead of personally risking his own neck. The resulting "final battle" between the Forces of Light and Darkness raged out of control until the only two kombatants left standing were a weakened and insane Raiden and Shao Khan nearly victorious. Before the timeline was altered, Shinnok, Shao Kahn, Raiden, Taven and the disembodied spirit of Liu Kang were the only survivors of Armaggedon.
In the reset timeline, Shinnok is a background figure until its revealed he is the true mastermind behind Shao Kahn's invasion of Earthrealm, having used his acolyte Quan Chi to instruct the warlord into merging the realms without victory in Mortal Kombat. Kahn was killed by Raiden during the final battle however most of Earthrealm's champions have been killed in action and Outworld was without an emperor. Shinnok and Quan Chi are poised to begin their invasion once he's set free, with Earthrealm and Outworld far more vulnerable to a hostile takeover from the Netherrealm than they were in the original timeline.
Two years after Shao Kahn's invasion of Earthrealm, Shinnok launches an invasion of his own with Netherrealm forces and the Brotherhood of Shadow. Shinnok personally goes to the Jinsei Chamber in order to taint Earthrealm's life force with death. He defeated and nearly imprisoned Raiden and Fujin within his amulet, only to be struck down by Johnny Cage. As an outraged Shinnok incapacitates Johnny's companions, the Earthrealm warrior holds him off and passes the amulet over to Raiden, who imprisons the fallen Elder God within his own amulet. Shinnok would spent the next 25 years dormant...